A ball screw is a device including a screw shaft and a nut member and has a function for converting rotating motion to linear motion or converting torque to thrust or vice versa. The ball screw attaining such function is used as a device for performing transfer and/or conversion of driving force in all machine or apparatus including general machines, machine tools, precise machines and so on.
For example, as a machine using such ball screw includes an injection molding machine. The injection molding machine is generally provided with a die fastening device and an injection device disposed above the die fastening device to be vertically movable, which utilizes the ball screw for effecting the transfer of the die fastening force in the die fastening device and an injecting motion in the injection device (for example, refer to the following Patent Publication 1).    Patent Publication 1: Japanese Unexamined Patent Application Publication No. 2004-98703